1. Field of the Invention
The invention relates to a plate for an electrode of an electrolytic condenser with an increased electrical output which can reach a value above 300,000 microfarads/dm.sup.2, and the invention also relates to a process for the manufacture of said plate.
The person skilled in the manufacture of electrolytic condensers, in particular those constituted of aluminium plates, knows that in order to obtain an increased output it is necessary to increase the surface area of the electrodes. This results in the application of all types of engraving to said plate.
Most frequently, the engraving is obtained by chemical or electrochemical attack which makes the surface of the plate rough and/or porous.
More recently, another technique has been used which consists in physically depositing a metal film on the plate in order to produce said porosity. This allows for an improvement in the output, whilst avoiding recourse to attack baths which use chemical products which are toxic or corrosive to varying degrees, or which are harmful to the environment.
In this coating process, as in the process for engraving, the plate obtained then undergoes a treatment known as chemical or electrochemical formation and/or stabilisation which uses an electrolyte and which is intended to prevent degradation of the surface state by the action of external agents, and which thus aims to preserve the output of the condenser over the passage of time.
In the coating domain, it is possible to cite, for example, U.S. Pat. No. 4,309,810 which discloses an electrolytic condenser in the form of a metal or plastics substrate coated by evaporation under vacuum with a deposit of either aluminium or tantalum or an alloy of several metals. The deposit obtained is porous and formed of a series of metal crystal columns; however, when a trace of oxygen is introduced into the deposit chamber under vacuum the deposited crystals decrease in size, and crosslinkage to a certain extent of said crystals results.
To be more exact, this patent claims: "A process for obtaining a condenser, comprising three stages:
implementation of a substrate with a surface, PA0 supply of a heat source with an anodisable metal coming from a thread spool, PA0 evaporation of said metal to form a metallic vapour, PA0 depositing said metallic vapour over the surface of said substrate at an angle of incidence of less than 60 degrees and in the presence of oxygen at a partial pressure not in excess of 10.sup.-4 Torr (0.0133 Pascal) in such a way as to obtain a porous metallic coating, PA0 anodising the surface of the porous coating, PA0 providing an electrical contact on said anodised surface, PA0 and winding said substrate bearing said anodised porous coating in the form of the condenser." PA0 from the point of view of composition, the deposit is not in the form of pure aluminium, but of a mixture wherein more than 50% by weight of the aluminium is in the form of oxide; PA0 from the point of view of structure, the deposit is constituted of grain agglomerates which form a porous matrix of oxide, and aluminium crystallites are dispersed in the grains unlike in the prior art where the deposit was constituted of an assembly of purely metallic crystals.
In one example of application, it is stated that an aluminium plate of thickness 10 micrometers is coated with a porous film which is 6 micrometers in thickness by evaporation of aluminium in a chamber under vacuum. After anodisation of the porous film in a solution with 4% by weight boric acid, and at 200 V formation voltage, the output of the aluminium plate per unit of surface area is 96 microfarads per dm.sup.2.
The constant problem in the domain of electrodes of electrolytic condensers, whether for the anode or cathode, is that of their electrical output per unit of surface area. In fact, the greater the specific output, the greater the possible reduction in size of the electrodes, even with total output.
In addition to reducing the cost, this makes it possible for the condensers to be reduced in size and for them to be used in applications where a reduction in bulk and weight is desirable.
Since the method of coating a substrate with aluminium and then following it by formation and/or stabilisation has made progress from the point of view of electrical output, the Inventor thought that by using this same method it would be possible to make considerable improvements and to reach output values far in excess of those reached hitherto, in particular by using much thinner coatings, thus at a much lower cost.
The Inventor found that this improvement necessitated the presence of a deposit of a special composition and structure which adheres properly to the substrate, this deposit also having to be suitably stable with the passage of time, in particular to external agents in such a way that the output is preserved with the passage of time.